This invention relates to a magnetic disk apparatus which employs a sector servo system as the tracking servo of a magnetic head having a pre-erase gap.
When recording new data on a flexible (floppy) disk, in place of previously-recorded (old) data, the new data is simply written over the old data, i.e., without first erasing the old data.
With recent increases in the recording capacity of floppy disks, much attention has been given to the problem wherein when new data is written over old data, noise is produced as a result of imperfect erasure, or overwrite modulation occurs. In the case of a perpendicular magnetic recording medium, in particular, which has a thick magnetic layer formed thereon, it is difficult to obtain a digital recording of a good quality when data is overwritten by a ring-shaped read/write (R/W) head.
To cope with the occurrence of overwrite modulation in this type of recording medium, a recording/reproduction system has been proposed which employs a magnetic head assembly having an erase gap of greater length and width than the R/W gap which it precedes. Such a recording/reproduction system is disclosed in Japanese Laid Open Patent Publication Nos. 61-39910 and 61-39911. The R/W gap and erase gap are integrally formed in the magnetic head assembly. According to the recording/reproduction system using this magnetic head assembly a data write region is erased by the erase gap prior to data write by the R/W gap. A floppy disk employing this system is described in a paper "Barium Ferrite Perpendicular Magnetic Recording Floppy Disk" in TOSHIBA REVIEW 1985 Vol. 40 No. 13, p.p. 1115-1118.
The head having such a pre-erase gap as described above is also known as a wide pre-erase head. This head has a structure as shown in FIG. 1. The width of a R/W gap 1 is 0.35 .mu.m, which ensures a recording density of 35 KBPI (bits per inch). An erase gap 2 with a width of 2 .mu.m or less is formed such that it precedes R/W gap 1 with respect to the rotational direction of the disk. The spacing between R/W gap 1 and erase gap 2 should preferably be even less, in consideration of disk formatting efficiency. On the other hand, if the spacing is too narrow, it becomes increasingly difficult to manufacture the head, and results in a decrease in the efficiency of the head. Accordingly, the spacing between those two gaps cannot be set too narrow, and so, is generally set to as little as 200 .mu.m. With a recording density of 35 KBPI, the 200 .mu.m spacing corresponds to about 35 bytes at the outermost track on a 3.5 inch floppy disk. When the erase gap and R/W gap are driven simultaneously, it becomes clear that the data region corresponding to the spacing between the erase gap and R/W gap cannot be erased. This necessitates the provision of an unused area or gap corresponding to the spacing between the erase and R/W gaps at a beginning portion of the data write region on the disk.
In the floppy disk soft sector track format disclosed in the aforementioned TOSHIBA REVIEW, the unused area is formed at the beginning of each of address and data fields in every sector, i.e., the preceding section of a sync area, as shown in FIG. 2. Stated another way, two unused areas are formed in each sector.
When an unused area is provided at the beginning of each of the address and data fields, then, as in the case of a commercially available magnetic disk apparatus using a tunnel-erase type magnetic head, even when the R/W gap and erase gap are driven simultaneously, accidental erasure of format data can be reliably prevented, thereby ensuring writing of data of a high quality. In practice, however, it is necessary to set the length of the unused area to more than 35 bytes, due to the possibility of errors in the manufacturing process, resulting in variations in the spacing between the R/W and erase gaps, as well as the possibility of uneveness in the rotation of disks.
A sector servo system, in which servo information is embedded in part of each sector of a disk, for the purpose of attaining a higher track density, and which utilizes this servo information for head positioning, is disclosed in U.S. Pat. No. 4,499,511 issued Feb. 12, 1985, U.S. Pat. No. 4,631,606 issued Dec. 23, 1986, and copending U.S. patent application Ser. No. 807,998 filed Dec. 12, 1985 and assigned to the same assignee as of this application. Greater recording capacity and higher density recording are made possible by applying this sector servo system to a floppy disk device using a magnetic head having a pre-erase gap, which is allowed to use a high density recording medium, such as a perpendicular magnetic recording medium. In the case of the sector servo system, the format of a floppy disk is of a hard sector type. Therefore, it is necessary to provide an unused area at the end portion of a data field in each sector, in addition to the aforementioned two unused areas, and a servo area in another area of the sector.
FIG. 3A illustrates an example of the hard sector format, using a SMD (small module disk) interface or an ESDI (enhanced small disk interface). In the hard sector format, unlike in the soft sector format, format data is transferred to a disk drive by a disk controller. A sector pulse is generated from a disk by the disk device (disk drive). In FIG. 3A, "PLO" stands for a sync area used for locking a phase-locked oscillator, while "EOR38 stands for an area used for representing the end of record.
When the pre-erase head system is applied to this hard sector format, it is necessary to provide three unused areas in one sector, as is shown in FIG. 3B; namely, those formed at the beginning portion of the address field and at the beginning and end portions of the data field. These unused areas are indicated as gap.sub.1, gap.sub.2, and gap.sub.3. In particular, the last unused area, gap.sub.3, is necessary to prevent erasure of a sector detection mark provided in gap.sub.1 that is formed at the beginning of each sector. The number of bytes corresponding to the spacing between the R/W gap and the erase gap varies between the innermost track and the outermost track. The number of bytes at the innermost track is largest, so that the length of each unused area is determined by taking into consideration the number of bytes corresponding to the head gap spacing at the innermost track.
Further, when the servo sector system is applied to this format, in addition to the three unused areas, a servo information area must be additionally provided between gap.sub.1 and gap.sub.2, as is shown in FIG. 3B. However, provision of this additional area significantly lowers the format efficiency of the disk. In particular, when the data storage capacity for one sector is 256 bytes, the data storage efficiency is below 50% and the reduction in the format efficiency cannot be neglected.